1. Field of the Invention
The present invention relates to a holographic recording medium written and read by a holographic storage device, and to a writing method therefor.
2. Description of the Related Art
Conventionally, as a storage device used in a computer and the like, storage devices are widely being used in which information is two-dimensionally written or read with respect to a recording medium by a magnetic or optical method. A hard disk is known as a storage medium using magnetism, and a CD or DVD is known as a storage medium using light. These storage media are remarkably advanced in a recording density to meet a demand on a large amount of capacity. Also, as a means for a larger amount of capacity, storage media using the holographic principle are under development.
The holographic storage devices read information recorded on the recording medium as holograms in a page unit to reproduce it. On the recording medium, coded information in a page unit is written as a pattern, for example, in which an index of refraction varies. The pattern is a hologram formed by interference between an object beam and a reference beam in a storage device. Also, in order to read the information from the recording medium, the reference beam alone is incident on the recording medium, is diffracted by the hologram pattern, and is received by a photoelectric transducer, such as a CCD or a CMOS. In this way, the written information can be reproduced. Such a holographic storage device is, for example, disclosed in Japanese Unexamined Patent Application Publication No. 2003-43904.
However, when the incidence angles of the reference beam with respect to the holographic recording medium are different from each other in writing and reading information, the reference beam is not diffracted by a pattern of the hologram, so that the written information cannot be read because the wave front of the object beam in writing the information is not reproduced. For this reason, when the information recorded on the holographic recording medium is read, very high precision is required with respect to the reference beam incident on the holographic recording medium, which leads to an increase in manufacturing costs.